This invention relates to greases and, more particularly, to lithium soap thickened greases.
Many mechanisms such as bearings, actuator screws, gauges, instruments, aircraft, vehicles, tanks, and other military equipment are required to perform well at very low temperatures, such as at -100.degree. F. Lubricating greases for such mechanisms, therefore, must perform well at low temperatures. It is desirable that such lubricating greases have outstanding oxidation resistance, good extreme pressure (EP) antiwear properties, superior pliability, and excellent stability at normal, as well as very low, temperatures.
Conventional polyalphaolefin (PAO) greases are economical and have many fine qualities, but are usually not reliable nor do they perform well at low temperatures, such as well below -40.degree. F., especially below -65.degree. F.
Conventional mineral oil based greases are also limited in their usefulness in low temperature applications. For example, greases made from paraffinic mineral oil often provide below average performance at low temperatures because of wax which is usually present in the grease. At temperatures below 0.degree. F., wax can crystallize out and render the grease hard and non-pliable. Dewaxing processes can reduce the wax level in paraffinic mineral oil but cannot eliminate it altogether. Naphthenic mineral oils have virtually no wax and have better low temperature flow properties, but do not give good flow properties at extremely low temperatures, such as -65.degree. F. to -100.degree. F. Also, naphthenic oils are more prone to oxidative and thermal degradation at high temperatures.
In the past two decades, diesters and other synthetic oils have been used as replacements of mineral oil in fluid lubricants and greases. Such diesters include dialkyl esters of dicarboxylic acids, such as di-2-ethyl hexyl azelate, di-isodecyl azelate, di-tridecyl azelate, di-isodecyl adipate, di-tridecyl adipate, and many others. Desirably, diesters have good low temperature flow properties and reasonably good resistance to oxidative breakdown. Unfortunately, however, diesters have very poor hydrolytic stability and will break down into two alcohol moieties and one dicarboxylic acid moiety when heated in the presence of water. The situation is made even worse when acidic or basic conditions are present since the hydrolytic breakdown of the diester is effectively catalyzed by acid or base. The above factors have traditionally made diesters a poor synthetic base oil choice for lithium soap thickened greases.
In lithium soap thickened greases, the metal base, usually lithium hydroxide or in its more commonly available form of lithium hydroxide monohydrate, is reacted with a fatty acid, usually 12-hydroxystearic acid, or with a fatty acid derivative, usually methyl 12-hydroxystearate or hydrogenated castor oil. This reaction is most often carried out in the base oil with water also being present. The water is added to act as a reaction solvent if the acid is used. If the fatty acid derivative is used, the water acts both as reaction solvent and reactant, the latter effect being necessary for the hydrolytic cleavage of the ester linkages in the methyl 12-hydroxystearate or the hydrogenated castor oil. When forming a grease thickener soap in a diester synthetic oil, the same conditions normally used to react the thickener components can cause hydrolytic breakdown of the diester. Resulting products can have little or no grease texture and such products are unsatisfactory as high performance lubricants.
Lithium 12-hydroxystearate greases have been successfully made in diester synthetic fluids. However, to do so requires extreme care in controlling the temperature/time profile during the formation of the thickener. The manufacturing procedure must maintain a delicate balance, forming the lithium 12-hydroxystearate without significantly hydrolyzing the diester oil. Correspondingly, such procedures are labor intensive and cumbersome. Even when successfully made, the resulting lithium greases will inevitably have some reaction products from unwanted hydrolytic breakdown of diester and subsequent neutralization of the formed dicarboxylic acid moieties by lithium hydroxide. An equivalent amount of unreacted 12-hydroxystearic acid will remain in the final grease after all the lithium hydroxide has reacted. This unwanted side reaction can cause a lower grease yield due to the considerably less thickening power of the dilithium salt of the dicarboxylic acid moiety of the diester. Yield of the lithium grease is also lowered since the grease cannot be heated to the melting point of lithium 12-hydroxystearate to improve certain properties of the grease without thermally degrading the grease. Melting and recrystallizing cannot be carried out in conventional lithium greases made in diester oils since an undesirable transesterification between free, unreacted 12-hydroxystearic acid and diester oil can occur at the required high temperature.
Another type of synthetic oil is polyol esters. Polyol esters have good oxidation and hydrolytic stability but are expensive, unreliable, and have mediocre performance.
Typifying some of the many types of prior art lubricating oils, greases, and additives, are those described in U.S. Pat. Nos. 3,622,512; 3,853,775; 3,876,550; 3,890,363; 4,514,312; and 4,536,308. These prior art lubricating oils, greases, and additives have met with varying degrees of success but generally do not perform well at low temperatures, especially at ultra low temperatures near -100.degree. F.
It is, therefore, desirable to provide an improved lithium grease which performs well at these low temperatures.